On Tue, Dec 12, 2017 at 05:37:59PM +0000, Russell King - ARM Linux wrote:
> On Tue, Dec 12, 2017 at 09:20:59AM -0800, Paul E. McKenney wrote:
> > The ARM implementation of arch_cpu_idle_dead() invokes complete(), but
> > does so after RCU has stopped watching the outgoing CPU, which results
> > in lockdep complaints because complete() invokes functions containing RCU
> > readers. This patch therefore uses Thomas Gleixner's trick of delegating
> > the complete() call to a surviving CPU via smp_call_function_single().
> >
> > Reported-by: Peng Fan <[email protected]>
> > Reported-by: Russell King - ARM Linux <[email protected]>
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > Tested-by: Tested-by: Fabio Estevam <[email protected]>
> > Cc: Russell King <[email protected]>
> > Cc: Ingo Molnar <[email protected]>
> > Cc: "Peter Zijlstra (Intel)" <[email protected]>
> > Cc: Michal Hocko <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > Cc: <[email protected]>
>
> As I just described in response to Fabio's testing, this doesn't solve
> anything if CONFIG_BL_SWITCHER is enabled. We could lose the unlock of
> a spinlock in the GIC code for sending the IPI. As I already said
> previously in our discussion (but I guess you just don't believe me):
Sorry, Russell, but most days I don't even believe myself. So it is
nothing personal, just one of the occupational hazards of being me.
> "2. there's some optional locking in the GIC driver that cause problems
> for the cpu dying path.
>
> The concensus last time around was that the IPI solution is a non-
> starter, so the seven year proven-reliable solution (disregarding the
> RCU warning) persists because I don't think anyone came up with a
> better solution."
>
> Using smp_call_function_single() invokes the IPI paths.
OK, another approach is to have the dying CPU simply set an in-memory
flag, which a surviving CPU polls for. There are of course any number
of ways of doing the polling loop.
So what bad thing happens when you use that approach?
Thanx, Paul
